1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and particularly, to an image forming apparatus and an image forming method capable of high precision image formation.
2. Description of the Related Art
In the related art, in an image forming apparatus able to form full color images on a recording medium, visible images of different colors (for example, yellow, magenta, cyan, black, and so on) are formed on plural photoconductive members (such as photoconductive drums), respectively, and these visible images are transferred to and superposed on an intermediate transferring member, or a recording medium directly to form a full color image.
In such an image forming apparatus, in order to form images with high precision, it is important for the visible images to be formed at specified image density. If the adhesion quantity of image visualizing agents, which result in the visible images, is not stable, the color tone may change, and the same image cannot be reproduced.
In addition, in order to form images with high precision, it is also important to eliminate position shift between the visible images during image transfer. If the position shift between the visible images occurs, it is difficult to obtain images with high precision.
To solve this problem, it has been proposed that in the image forming apparatus able to form full color images, registration marks be formed from the image visualizing agents of different colors on moving parts such as a paper conveyance belt, so that the image position shift is corrected based on the position information of the registration marks. For example, Japanese Laid-Open Patent Application No. 63-300261 discloses such a technique (hereinafter, referred to as “reference 1”).
Usually, correction control employing the registration marks, as disclosed in reference 1, is performed when the power of the image forming apparatus is switched on, or when the image forming apparatus is initialized as the total number of printed documents reaches a preset value. In other words, the period of the correction is long.
Therefore, for an image forming apparatus having a low printing speed, it is possible to maintain image quality even when the correction period is long, but for an image forming apparatus which forms full color images on a web-like recording medium (specifically, a long continuous belt-like recording medium, such as, continuous paper), for example, which is conveyed at a speed over a few tens inches per second, since the position shift is associated with the tolerance of component parts (here, the tolerance is defined to be the difference between allowed maximum and the minimum errors of a workpiece to be machined), the position shift is apt to be accumulated compared to the low-speed image forming apparatus.
In addition, although an image forming apparatus able to form full color images on continuous paper is disclosed in reference 1, since the registration marks are formed at the two edges of the continuous paper, it is necessary to cut the two edges of the continuous paper to remove the registration marks after the images are formed; thereby, the efficiency is poor.